This invention relates to optical waveguide transmission devices and more particularly to optical waveguide devices suitable, for example, for employment in optical multiplexer and demultiplexer, and router applications employing Wavelength Division Multiplexing (WDM) which may find application in optical fiber based communication networks.
Increasingly, state-of-the-art optical transmission devices for wavelength division multiplexing utilize waveguide gratings. In a typical device structure a waveguide grating may be placed between input and output slab waveguides. Multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface. The grating itself may include a number of different physical length waveguides with a constant length increment. This results in a wavefront rotating with the wavelength change. The grating can be also used to focus an output waveform and to collimate an input waveform. For this purpose the ends of the grating may be located on a circular surface provided by each slab waveguide, at which the waveguides are located close to each other to assure strong coupling needed for high efficiency. However, in the grating itself, coupling between individual waveguides is not desirable, so that the waveguides are spaced relatively far apart from each other. This implies a relatively large grating structure. In an optical multiplexer, multi-wavelength light from a point source, the end of the input waveguide, is transmitted through the grating and different individual wavelengths are focused on different points of the output waveguide surface.
Close coupling between waveguides, at the interface between the slab waveguide and the grating, has typically been considered essential to the efficiency of the device. To alleviate concerns about resulting aberrations, it has been proposed to position the foci of interface arcs between the input waveguide and the slab region, and the slab region and the waveguide grating, some distances from the arcs themselves. This approach of positioning the focal points of the arc boundary (center of the interface arc) away from the opposite arc, originating from the desire to maintain strong coupling, has continued. The emphasis on strong coupling was consistent with grating based multiplexer designs for handling a small number of channels (for example, 1xc3x974 to 1xc3x978) and high efficiency, i.e. low transmission loss.
Another desideratum in multiplexer design is a compact device, and in particular a compact waveguide grating section. This is driven by cost reduction associated with reduced size, as well as lower sensitivity to material non-uniformities. One approach addressing the compactness issue has been to introduce a reflector in the grating section. Possible reflecting arrangements include waveguide Bragg reflectors and mirrors, including mirrors with stepped surfaces. However, such approaches have only partially addressed the overall issue of compactness and transmission efficiency.
According to the present invention, an optical waveguide device structure comprises a slab waveguide having oppositely disposed arcuate first and second end surfaces. The first arcuate end surface provides ports for coupling to input and output waveguide sections. The slab waveguide has a dielectric layer waveguide structure which supports essentially two-dimensional light wave propagation by lateral diffraction from one of said ports to the second arcuate end surface. Laterally spaced apart tapered optical waveguide sections extend radially outwards from the arcuate second end surface of the slab waveguide, and have wider ends to collect light waves propagated from the first arcuate end surface to the second arcuate end surface and to direct the collected light waves to narrower ends of the tapered waveguide sections which provide optically isolated transmission paths for the collected light waves.
Advantageously, the arcuate first end surface has a radius of curvature originating on the arcuate second end surfaces, and said arcuate second end surface has a radius of curvature originating on the first arcuate end surface.
A preferred feature of the invention is that the tapered waveguide sections and the grating waveguides are laterally spaced apart sufficiently to provide optically isolated transmission paths for light waves between said free space region and said reflector surface. The tapered waveguide sections can be suitably configured to enhance collection of light waves transmitted across the free space region and to separate the collected light waves into optically isolated paths, even at the interface between the wider ends of the tapered waveguide sections and the free space region.
The dielectric layered waveguide structure may be defined by high refractive index core material sandwiched between low refractive index cladding material.
In a particular application of the invention, the narrower ends of the tapered waveguide sections may be coupled to a reflector surface by optically isolated waveguides of an optical waveguide grating.
The invention is advantageous in enabling a compact free space region to be defined by the slab waveguide structure.
Structures embodying the invention may be used in implementing a WDM device capable of handling a larger number of channels (e.g. 40 channels with a channel separation as close as 50-100 GHz) while giving rise to low cross talk between adjacent channels (e.g. less than 50 db) and significantly reduced variation of transmission efficiency across the response spectrum of the device. An important contribution to the improved functionality device is the decoupling of the waveguide sections along the length of the waveguide grating, and of particular importance, along the lengths of the tapered waveguide sections at the slab waveguide end. The particular mode of deployment of the tapered waveguide sections significantly contributes to this desired separation without adversely affecting overall transmission efficiency.